The present embodiments relate to positron emission tomography (PET). PET acquisition requires scans of long duration, and a significant magnitude of patient motion during the scan is sometimes unavoidable. For example, the pattern caused by breathing may result in a relatively large displacement of organs and consequent blurring of clinically relevant PET features in regions affected by the motion.
To limit the effects of motion, the PET data is separated by phases of the breathing cycle. The breathing pattern may be monitored either by external devices or by tracing the movement of objects in the list mode file domain of the PET data. Once the breathing pattern is established, PET data is separated into gates according to the phase or amplitude of respiration. Each gate represents a particular (quasi) frozen phase of motion. The gate with minimally integrated motion is chosen to reconstruct the PET image. While the motion artifact is suppressed, image quality suffers from a greater amount of noise due to reconstruction from less than all the PET data.
To achieve motion artifact suppression and approach maximum signal-to-noise ratio (SNR), each gate's data is individually reconstructed and registered to one of the reconstructions in the image domain. The aligned reconstructions (or images from the reconstructions) are then averaged. Another approach is to reconstruct one image from all available data through the incorporation of the field of motion into the reconstruction process (i.e., repeated each iteration of the reconstruction or in the forward and backward projection loop). This approach is fairly computationally extensive and also requires assessment of the motion phase for correction factors, such as attenuation.